This invention relates to an underwater ultrasonic wave transmitter usable for long-distance sonars and in the investigation of oceanic resources, which operates at high-power at low-frequencies. The use of low-frequency ultrasonic waves for sonars and the like is advantageous because of the small propagation loss as compared with high-frequency ultrasonic waves. Conventional transmitters adapted to radiate high-power ultrasonic waves in water include the electrodynamic transmitter and the piezoelectric transmitter, which are widely known. The electrodynamic transmitter is capable of great displacement but it has small generating power. Therefore, it is very difficult to obtain a miniaturized transducer for low-frequency ultrasonic waves. The piezoelectric transmitter uses a piezoelectric ceramic material of zircon-lead titanate as an electromechanical energy-converting material. Since the acoustic impedance of the piezoelectric ceramic material is about 20 times as high as that of water, or more, the generating power of this material is very high, but this material is incapable of being displaced so as to meet the requirements of media displacement during the acoustic radiation of the transmitter. The acoustic radiation impedance per unit radiation area of the piezoelectric ceramic material decreases at a high rate as the frequency of the ultrasonic waves to be transmitted decreases. Thus, it is necessary that low-frequency acoustic radiation be carried out with the displacement of the piezoelectric ceramic material further enlarged, so as to improve the efficiency of the acoustic radiation.
The known high-power transmitters for the low-frequency band (not more than 3 KHz) include the bendable transmitter utilizing piezoelectric discs, as shown in FIG. 1, which transmitter is disclosed in, for example, R. S. Woolette, "Trends and Problems in Sonar Transducer Design", IEEE Trans. on Ultrasonics Engineering, pp 116-124 (1963), and the flextensional transmitter which uses an elliptical shell, as shown in FIG. 2, which transmitter is disclosed in, for example, G. Brigham and B. Grass, "Present Status in Flextensional Transducer Technology", J. Acoust. Soc. Am., vol. 68, No. 4, pp 1046-1052 (1980).
The bendable transmitter shown in FIG. 1 generally uses circular bimorphous oscillators. Referring to FIG. 1, reference numeral 10 denotes plates of a piezoelectric ceramic material (zircon lead titanate), and 11 indicates metal plates of nickel or stainless steel. The plates 10, 11 form bimorphous oscillators, which are used as acoustic radiators. Reference numeral 12 denotes a cavity, and 13, a housing. However, in the transmitter shown in FIG. 1, each of the bimorphous oscillators is actually obtained by bonding a plurality of ceramic segment plates in a mosaic pattern to the metal plate 11, since a one-piece piezoelectric ceramic plate 10 of the large surface area required cannot be obtained. Namely, since one-piece ceramic plates of large area are not available, thus medium-displacement capability of this transmitter is not sufficiently high, so that this transmitter is not suitable for the case where a high-power transmitter is required. Even if one-piece piezoelectric ceramic plates of a large area could be obtained, the flexure compliance of the bimorphous oscillator becomes considerably large due to the construction thereof, and a great increase in the medium-displacement capability of the transmitter cannot be expected.
The flextensional transmitter shown in FIG. 2 uses a kind of displacement-enlarging mechanism, by which, when an active columnar member 20 consisting of a piezoelectric ceramic material is expanded in the direction of the longer axis thereof, an elliptical shell 21 contracts as shown by the arrows in the drawing. The degree of displacement is several times higher than that of the displacement of the columnar member 20. (The illustrative arrows are drawn around only 1/4 of the circumferential portion of the elliptical shell.) Since this transmitter uses an elliptical shell as an acoustic radiator, a structure which is far more rigid than that using bimorphous discs can be obtained. Therefore, it is said that the transmitter of FIG. 2 is better suited for the high-power transmission of ultrasonic waves than the transmitter of FIG. 1 which uses bimorphous discs.
The resonant frequency of the flextensional transmitter shown in FIG. 2 is two or more times higher than that of the elliptical shell 21 since the stiffness of the active columnar member 20 is considerably high as compared with that of the shell 21. Namely, unless the resonant frequency relative to the flextensional mode of the elliptic shell 21, which has predetermined dimensions, is reduced considerably, a reduction in the frequency and dimensions of the flextensional transmitter cannot be achieved. It has been required that the resonant frequency of the shell in the flextensional transmitter be further reduced. However, for the following reasons it has not been possible to reduce the frequency and dimensions of the elliptical shell.
In order to describe the operation of the device, a quadrant thereof is shown in FIG. 3, in which the longer axis, shorter axis and thickness of the shell are taken in the x-axis, y-axis and z-axis directions, respectively. Let (a, O) be the point at which the center of the thickness of the elliptical shell crosses the x-axis, and let (O, b) be the point at which the y-axis crosses the same center. Namely, let a and b equal the longer diameter and shorter diameter, respectively, of the elliptical shell. If the active columnar member 20 is expanded beyond point P in the positive x-direction by .epsilon., the shell is displaced beyond point Q in the negative y-direction by a distance several times greater than .epsilon., due to the displacement-enlarging mechanism of the elliptical shell, so that the shell as a whole draws the medium in. On the other hand, when the active columnar member contracts, the shell as a whole works in the medium-displacement direction. In this transmitter, a cross section of the elliptical shell, which is obtained by cutting the shell with a plane including the x-axis, is displaced in parallel with the x-axis, and the quantity of rotary displacement thereof around the z-axis is zero. Therefore, the movement of the shell is restricted to the extent corresponding to the quantity of prohibited rotary movement thereof around the z-axis, and the resonant frequency of the shell increases. In the flextensional transmitter, it is hard to reduce the resonant frequency of the shell for these reasons, and, hence, it is very difficult to reduce the frequency and dimensions of the transmitter.
It is, of course, possible to attempt changing the shape and thickness of the elliptic shell so as to reduce the frequency and dimensions of the transmitter.
When the shape of the elliptic shell is varied, the resonant frequency of the shell certainly decreases in inverse proportion to b/a, i.e., as the shape of the shell is set more similar to a circle. However, in this case, as b/a is increased, the displacement-enlargment rate decreases greatly in comparison with the frequency. Therefore, the merits of changing the shape of the shell to miniaturize the shell are lost. It has also been ascertained that, when the thickness of the shell is reduced, the resonant frequency of the transmitter decreases. However, in this case, the medium-displacement capacity and the water pressure-resisting characteristics of the shell are greatly deteriorated.